1. Field of the Invention
The present invention relates to methods and apparatus for determining phase ambiguities in ranging and navigation systems and, more particularly, to techniques for rapidly and accurately detecting cycle slips in pseudorange measurements based on the carrier phase of navigation signals.
2. Description of the Related Art
Navigation systems such as the Global Positioning System (GPS) enable users to determine their present position based on received navigation signals. For example, the GPS architecture includes a constellation of twenty-four satellites that orbit the earth twice a day. The orbits of the GPS satellites are chosen so that navigation information can be provided to users regardless of the time that the user requests information and regardless of the user""s position on the earth""s surface. This information contains a navigation message that includes satellite position and satellite clock drift information.
The user""s GPS receiver operates by performing a radio-ranging calculation which involves acquiring the encoded signals transmitted by each GPS satellite and making pseudorange measurements. Pseudorange is the distance from the user to the source of the signal (i.e., to GPS satellites), with some errors included in it, such as satellite clock bias, etc. Using trilateration techniques, pseudorange measurements received from several satellites are processed in real time to provide the best estimate of the user""s position (latitude, longitude, and altitude), velocity, and system time.
There are essentially two types of pseudorange measurements. One type is code-based and involves tracking the code (modulation) of the GPS signal to determine the pseudorange. The advantage of code-based pseudorange measurements is that they are absolute pseudorange measurements: the results of the measurements differ from the true pseudorange by the measurement noise only. The disadvantage of code-based pseudorange measurements is that the measurement noise may be large enough to adversely affect the accuracy of the position estimation.
The other type of pseudorange measurement is carrier-based and involves tracking the carrier phase of the GPS signal. Phase measurements of the carrier signal typically have much lower noise, but these phase measurements are offset from the absolute value of the pseudorange by some unknown integer number of wavelengths. Typically, this number, called the integer phase ambiguity or simply xe2x80x9cinteger ambiguityxe2x80x9d, can be maintained constant from one measurement time epoch to another, though a receiver may occasionally suffer from so-called cycle slips, which change the integer ambiguity in an unpredictable way. For example, the receiver, which continuously tracks the carrier phase, may momentarily lose track of the phase and, upon reacquisition, may report the phase as being several carrier cycles (wavelengths) greater or less than the phase just prior to loss of track, resulting in a sudden jump (i.e., a cycle slip) in the reported value of the carrier-based pseudorange measurement. Thus, phase measurements per se are useless, unless there is a method to determine and eliminate the integer ambiguity bias, including detection of cycle slips.
A number of techniques exist for correcting integer ambiguities in GPS measurements, each having certain shortcomings. Smoothing techniques typically involve averaging and combining code measurements with phase measurements to determine the integer phase ambiguity. However, conventional smoothing techniques require long start-up times after satellites appear in view or after a cycle slip.
Motion-based techniques employ time differences to eliminate integer bias. Such techniques have a number of drawbacks. Specifically, motion-based techniques cannot be applied in real time, and precision may suffer at times due to poor satellite geometry. Further, motion-based techniques for determining ambiguities poorly guard against cycle slips.
Other approaches to resolving integer ambiguities in GPS measurements include integer space searching using signals from multiple satellites and/or current information on the position, such as described in U.S. Pat. No. 5,296,861, the disclosure of which is incorporated herein by reference in its entirety. Such search techniques typically require multiple signals and additional information on position (e.g., attitude, etc.), which potentially create a chicken-and-egg problem. Also, such techniques can be very computationally intensive for long baselines.
Thus, GPS navigation systems, as well as other types of communication links which may benefit from determination of pseudorange, would be significantly enhanced by improved techniques for determining integer phase ambiguities in carrier-based pseudorange measurements, including the capability to rapidly and accurately detect cycle slips.
Therefore, in light of the above, and for other reasons that become apparent when the invention is fully described, an object of the present invention is to more accurately determine the value of integer phase ambiguities of pseudorange measurements that are based on the carrier phase of navigation signals.
A further object of the present invention is to improve detection of cycle slips in carrier-based pseudorange measurements and to avoid false detection of cycle slips, thereby minimizing the impact of cycle slips on the accuracy of the estimated pseudorange.
Yet a further object of the present invention is to rapidly detect and respond to cycle slips and to rapidly establish the integer ambiguity value under start-up conditions.
A still further object of the present invention is to accurately estimate uncertainty in selection of integer ambiguity values to facilitate appropriate weighting of pseudorange estimates in tracking systems.
Another object of the present invention is to more accurately estimate the pseudorange between and receiver and a signal source.
Yet another object of the present invention is to more accurately estimate the position of a receiver receiving navigation signals.
Still another object of the present invention is to provide enhanced ranging and position determining capabilities in navigation systems such as GPS and other communication systems which can benefit from an accurate determination of range between a transmitter and a receiver.
The aforesaid objects are achieved individually and in combination, and it is not intended that the present invention be construed as requiring two or more of the objects to be combined unless expressly required by the claims attached hereto.
The present invention provides an improved technique for determining the integer value of phase ambiguities, known as xe2x80x9cinteger ambiguitiesxe2x80x9d, of pseudorange measurements that are based on the carrier phase of a received navigation signal, i.e., carrier-based pseudorange measurements. The technique relies on the principles of Bayes""s theorem to estimate the probability that each of a group of candidate integer ambiguity values is the true, correct value of the integer ambiguity at the measurement time epoch. After computing the estimated probability for the set of candidate values, the candidate value whose probability is greatest is selected as the value of the integer ambiguity. The probability estimates are computed based on a number of factors, including the code-based pseudorange measurement and the carrier-based pseudorange measurement at the present time epoch, the estimated variability of these measurements, the integer ambiguity probabilities computed at the preceding time epoch, and the probability of a cycle slip from the preceding time epoch to the present time epoch. A cycle slip is detected whenever the value of the integer ambiguity changes from a preceding value.
In comparison to smoothing techniques conventionally used to track the pseudorange, the Bayesian estimation approach of the present invention more accurately determines values of the integer ambiguity of the carrier cycle of the carrier-based pseudorange measurements, particularly at start-up and after cycle slips, and detects and responds to cycle slips much more rapidly.
The present invention also permits estimation of the uncertainty of the selected value of the integer ambiguity based on the estimated probability of the selected integer ambiguity value and estimated probabilities of neighboring integer ambiguity values. The uncertainty estimate can be used in a geographic position tracker, such as a Kalman filter, to appropriately weight the resulting pseudorange estimate in updating the tracked geographic state.
The techniques of the present invention can provide enhanced ranging and position determining capabilities in navigation systems such as GPS and other communication systems which can benefit from an accurate determination of range between a transmitter and a receiver.
The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following definitions, descriptions and descriptive figures of specific embodiments thereof wherein like reference numerals in the various figures are utilized to designate like components. While these descriptions go into specific details of the invention, it should be understood that variations may and do exist and would be apparent to those skilled in the art based on the descriptions herein.